Variations in hardness and microstructure in cartridge cases at annealing

Sammanfattning: The thesis work is performed at Norma Precision AB, a company that manufactures ammunition for hunting and competitive shooting. Two types of cases called 300 Winchester Magnum and 308 Winchester are examined on how their hardness were prior to and after the two annealing’s of the neck they experience during the manufacturing. The cases vary in hardness and an investigation will be made to draw connections to the microstructure of the cases in order to understand the variations and possibly avoid the variations in the future. The cases were selected at different days, the 300 Winchester Magnum were chosen at three different days with some properties varying and the 308 Winchester were taken at two different days. At each day twenty cases were selected, five prior to the first neck annealing, five after the first neck annealing, five after the shaping and calibrating of the cases and the final five after the second neck annealing. The selected cases were first measured by a hardness test, where they were measured at 1mm, 3mm and 6mm from the nuzzle and five times around the case at each measurement. After that, the variations were analyzed and the two most varying cases from each day and step were taken into the laboratory at Karlstad University. In the laboratory, the cases were measured by a microhardness tester. The cases were measured at even distances between them all throughout the cases to make a hardness profile. The next step was to etch the cases and examine them in an optical microscope, where the grain size was measured in micrometer and ASTM. The results from the regular hardness measurements and the microhardness tests showed similar tendencies. Variations was found in each step and each day during both hardness measurements the two types of cases behaved similarly. The grain size results showed that after annealing, the grain size increased. Although there was no clear relation between grain size and hardness since the results varied, some cases with higher hardness had a smaller grain size than a case with lower hardness. Using the grain size of the brass could work to get an idea of the hardness, but to get a sincerer perception of the material properties, more properties needs to be investigated.